Phenol and cyclohexanone are important products in the chemical industry and are useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, a common route for the production of phenol is the three-step Hock process via cumene. In the first step of the process benzene is alkylated with propylene in the presence of an acidic catalyst to produce cumene. The second step comprises oxidation, preferably aerobic oxidation, of cumene to the corresponding cumene hydroperoxide. The third step comprises cleavage of the cumene hydroperoxide, usually in the presence of a sulfuric acid catalyst, into substantially equimolar amounts of phenol and acetone, a co-product.
It is known that phenol and cyclohexanone can be co-produced from cyclohexylbenzene in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is provided in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. This patent reference also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
In the cumene-based Hock process, reaction product effluent from the cumene oxidation step is first concentrated to greater than 80 wt % by removing unreacted cumene under vacuum, and the resultant concentrate is then sent to the cleavage reactor. In addition to the hazards associated with handling concentrated hydroperoxide, the cleavage poses safety concerns due to the rapid and highly exothermic nature of the reaction. Further, significant amounts of by-products may be generated from the concentrated oxidation products. In practice, therefore, the concentrated cumene hydroperoxide is often diluted with solvents, such as acetone, in order to better manage the heat of reaction and to control by-product formation. For example, U.S. Pat. No. 5,254,751 discloses a method of producing phenol and acetone by decomposing cumene hydroperoxide in a non-isothermal manner in the presence of excess acetone whereby the molar ratio of acetone to phenol in a decomposition reactor is from about 1.1:1 to 1.5:1.
The process for making phenol from cyclohexylbenzene differs from the cumene process in several respects. Firstly, oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide is much more difficult than oxidation of cumene and requires elevated temperatures and the use of a catalyst, such as N-hydroxyphthalimide (NHPI). As a result, the cyclohexylbenzene oxidation effluent is also generally at elevated temperatures so that cooling this stream back to ambient temperature would incur additional operating cost. Also, in view of the high boiling point of cyclohexylbenzene, concentration of the cyclohexylbenzene hydroperoxide by evaporation of the residual cyclohexylbenzene is much more difficult. In addition, the cleavage chemistry for cyclohexylbenzene hydroperoxide is much more complicated than that for cumene hydroperoxide, particularly since more routes for by-product formation exist with cyclohexylbenzene hydroperoxide cleavage. Moreover, cyclohexanone is much more prone to acid-catalyzed aldol condensation reactions than acetone so that significant yield loss is possible unless the cyclohexylbenzene hydroperoxide cleavage is closely controlled.
There are other disadvantages of using sulfuric acid for cyclohexylbenzene hydroperoxide cleavage: 1) sulfuric acid is corrosive, especially in the presence of water, requiring expensive materials for reactor construction; 2) sulfuric acid needs to be neutralized before product separation and distillation, which requires additional chemicals such as phenate, caustics, or organic amines; and 3) the salt generated from neutralization requires separation and disposal and the waste water needs to be treated. Therefore, there are strong incentives to replace sulfuric acid with a heterogeneous cleavage catalyst that eliminates these drawbacks.
U.S. Pat. No. 4,490,565 discloses that zeolite beta can be an effective replacement for sulfuric acid in the cleavage of cumene hydroperoxide and that the yields, conversions and selectivities are generally superior to those produced by the use of the large pore zeolites X and Y. In U.S. Pat. No. 4,490,566, similar improvements over the large pore zeolites X and Y are reported with intermediate pore size zeolites, such as ZSM-5. In contrast, in an article entitled “Efficient Cleavage of Cumene Hydroperoxide over HUSY zeolites: The role of Bronsted activity”, Applied Catalysis A: General, 336 (2008), pages 29-34, Koltonov et al. report that cumene hydroperoxide readily undergoes decomposition over HUSY zeolites of high (15 to 40) Si/Al ratio with good selectivity to phenol and acetone and with efficiency even comparable to that of sulfuric acid. Despite or possibly because of these varying recommendations, most commercial processes for the cleavage of cumene hydroperoxide continue to use sulfuric acid as the catalyst.
In addition, International Patent Publication No. WO2012/145031 discloses that large pore zeolites of the FAU type having a unit cell size of less than 24.50 Åexhibit a unique combination of high activity and high selectivity activity for the conversion of cyclohexylbenzene hydroperoxide into phenol and cyclohexanone.